1. Field of the Invention
The present invention relates to a compensation circuit of an organic light emitting diode, in particular to the compensation circuit capable of maintaining a stable brightness of the organic light emitting diode.
2. Description of the Related Art
Active-matrix organic light-emitting diode (AMOLED) display device featuring thin thickness, light weight, self-luminescence, low driving voltage, high efficiency, high contrast, high color saturation, quick response speed, and high flexibility is considered as the most promising rising display technology after the development of thin film transistor liquid crystal display device (TFT-LCD).
Since the brightness of the organic light emitting diode (OLED) components depends on the magnitude of current, it is necessary to control the current accurately in order to control pixel brightness accurately, thus incurring a much higher level of difficulty than the conventional TFT-LCD that controls the pixel brightness by controlling the voltage level of writing in pixels only.
In fact, the AMOLED also encounters many problems. With reference to FIGS. 1 and 2 for schematic circuit diagrams of a P-type AMOLED pixel circuit without compensation and an N-type AMOLED pixel circuit without compensation respectively, the current IOLED of an OLED is a current obtained by converting the data voltage VData by using a thin-film transistor (TFT) which is the T200 as shown in FIGS. 1 and 2 in a saturated area, and its formula is IOLED=K(VGS−VTH)2. After the AMOLED has been used for a long time, the VTH of the TFT will increase, and the carrier mobility will decrease, so that the IOLED will drop to cause a brightness attenuation of the AMOLED.
Due to the ageing and deterioration of the OLED, the cross voltage of the OLED will rise and the light emitting efficiency of the OLED will drop gradually after a long time of operation. The rising cross voltage of the OLED may affect the operation of the thin film transistor. The N-type thin film transistor is used for example. If the OLED is coupled to a source of the thin film transistor, and the cross voltage of the OLED rises, then the voltage source between the gate and source of the thin film transistor will be affected directly, and the current passing through will be affected directly, too. As to the light emitting efficiency, a long-time operation will cause the ageing and deterioration of the light emitting efficiency. An expected brightness cannot be achieved, even if the same current is passed through. The drop of the light emitting efficiency for three colors such as red (R), green (G) and blue (B) is different from one another, so that a color shift issue arises, and this problem is not a problem that cannot be solved easily, since the material cannot be improved easily.
As the size of the panel increases, the signal line becomes longer, and the internal resistance becomes increasingly more significant, the uniform brightness of the panel will be affected. This phenomenon is called an I-R Drop. With reference to FIG. 3 for the schematic view of the I-R Drop, the VDD and VSS signal lines will produce a voltage difference by the internal resistance effect, and different currents are passed through different pixel positions of an AMOLED panel. As a result, the uniform brightness of the panel will be affected.